<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script>
        /* 遍历数组 */
        var arr3 = ['aa', 'bb', 'cc']
        console.log(arr3.length)
        for(var i=0; i<arr3.length; i++){
            console.log(arr3[i])   //遍历数组要从 0 开始
                                   //如果是 <=3 会输出[3]对应的值，是不存在的，就显示为 undefined
        } 
    </script>
</head>
<body>
    
</body>
</html>